Effect of Apolipoprotein E and Alzheimer’s disease related pathologies
The apolipoprotein E gene (APOE) is the strongest genetic risk factor for AD. One copy of the ε4 isoform of APOE increases AD risk by ~3.7 fold whereas two copies of the ε4 isoform increases risk by ~12-fold relative to the ε3 allele. One copy of the ε2 allele of APOE decreases AD risk by ~0.6. The ε4 allele also increases risk for CAA. While there are several ways that apoE may be contributing to AD risk, our data demonstrates, along with data from other labs, that a major mechanism by which apoE contributes to AD risk is via apoE’s effects on both Aβ clearance and aggregation. Using mouse models, we showed that apoE isoforms modulate Aβ deposition in a pattern very similar to that seen in humans. We found that apoE isoforms directly modify endogenous soluble ISF Aβ clearance (but not synthesis) in the brain ISF in vivo with Aβ clearance being in the order E2>E3>E4. This suggests that the reason for the apoE-isoform specific effects on Aβ deposition are due to differential effects of apoE isoforms on soluble Aβ clearance. We also found that the LDL receptor and ABCA1 strongly influence apoE levels and Aβ deposition in the brain via distinct mechanisms. Recently, we have also found that ApoE strongly influences tau-mediated neurodegeneration via a mechanism in part linked to effects on the brain’s innate system. This finding provides novel insights into the links between apoE and neurodegenerative diseases.
Kim J, Castellano JM, Jiang H, Basak JM, Parsadanian M, Pham V, Mason SM, Paul SM, Holtzman DM. Overexpression of low-density lipoprotein receptor in the brain markedly inhibits amyloid deposition and increases extracellular Abeta clearance. Neuron. 2009 Dec 10;64(5):632-44. PMCID: PMC2787195
Castellano JM, Kim J, Stewart FR, Hong J, DeMattos RB, Patterson BW, Fagan AM, Morris JC, Mawuenyega KG, Cruchaga C, Goate AM, Bales KR, Paul SM, Bateman RJ, Holtzman DM. (2011) Human apoE Isoforms Differentially Regulate Brain Amyloid-β Peptide Clearance. Science Translational Medicine 29;3(89):89ra57. PMCID: PMC3192364
Huynh TV, Liao F, Francis CM, Robinson GO, Serrano JR, Jiang H, Roh J, Finn MB, Sullivan PM, Esparza TJ, Stewart FR, Mahan TE, Ulrich JD, Cole T, Holtzman DM. Age-Dependent Effects of apoE Reduction Using Antisense Oligonucleotides in a Model of β-amyloidosis. Neuron. 2017 Dec 6;96(5):1013-1023.e4. doi: 10.1016/j.neuron.2017.11.014. PMCID:PMC5728673
Shi Y, Yamada K, Liddelow SA, Smith ST, Zhao Z, Luo W, Tsai RM, Spina S, Grinberg LT, Rojas JC, Gallardo G, Wang K, Roh J, Robinson G, Finn MB, Jiang H, Sullivan PM, Baufeld CB, Wood MW, Sutphen C, McCue L, Xiong C, Del-Aguila JL, Morris JC, Cruchaga C, Fagan AM, Miller BL, Boxer AL, Seeley WW, Butovsky O, Barres BA, Paul SM, Holtzman DM (2017) ApoE4 markedly exacerbates tau-mediated neurodegeneration in a mouse model of tauopathy. Nature 2017 Sep 28;549(7673):523-527. PMCID:PMC5641217
Antibodies targeting amyloid-β and ApoE
Active and passive immunization against the Aβ peptide remains a potential future therapy to most likely delay the onset/prevent and possibly treat very mild dementia in AD. My laboratory has been involved in utilizing animal models of Aβ deposition to demonstrate that passive administration with certain antibodies to Aβ and to apoE have potential as a therapy against AD, characterizing the mechanism of action of different anti-Aβ and anti-apoE antibodies, and demonstrating the potential utilization of antibodies for diagnostic purposes in AD. Some of our work highlighted below was the first demonstration that an antibody to soluble forms of Aβ can both decrease Aβ deposition in the brain but also increase plasma Aβ that is derived from the brain. This antibody, called m266 can also bind soluble forms of Aβ in the CNS after peripheral administration. The antibody was subsequently humanized by Eli Lilly and is now called Solanezumab. In 2016, it did not hit its primary endpoint in a phase III trial in mild dementia; however, it remains in two prevention trials (DIAN-TU and A4). More recently, we have found that a non-lipidated form of apoE is present in amyloid plaques and that an antibody to this form of apoE removes amyloid plaques similarly to several anti-Aβ antibodies.
DeMattos RB, Bales KR, Cummins DJ, Dodart J-C, Paul SM, Holtzman DM. (2001) Peripheral anti-Aβ antibody alters CNS and plasma Aβ clearance and decreases brain Aβ burden. Proceedings of the National Academy of Science USA 98: 8850-8855:10.1073/pnas.151261398.
DeMattos RB, Bales KR, Cummins DJ, Paul SM, Holtzman DM. (2002) Brain to plasma amyloid-β efflux: Ameasure of brain amyloid burden in a mouse model of Alzheimer’s disease. Science 295:2264-2267.
Kim J, Eltorai AE, Jiang H, Liao F, Verghese PB, Kim J, Stewart FR, Basak JM, Holtzman DM. (2012) Anti-apoE immunotherapy inhibits amyloid accumulation in a transgenic mouse model of Aβ amyloidosis. J Exp Med. 209(12):2149-56. PMCID: PMC3501350
Liao F, Li A, Xiong M, Bien-Ly N, Jiang H, Zhang Y, Finn MB, Hoyle R, Keyser J, Lefton KB,Robinson GO, Serrano JR, Silverman AP, Guo JL, Getz J, Henne K, Leyns CE, Gallardo G, Ulrich JD, Sullivan PM, Lerner EP, Hudry E, Sweeney ZK, Dennis MS, Hyman BT, Watts RJ, Holtzman DM. (2018) Targeting of nonlipidated, aggregated apoE with antibodies inhibits amyloid accumulation. J Clin Invest. 209(12):2149-56. PMCID: PMC3501350
Synaptic activity and amyloid-β
We found that levels of soluble, monomeric Aβ peptide in the brain in vivo, in both mice and humans, are directly regulated by synaptic activity, specifically, by synaptic vesicle release. This finding, together with our additional papers below, strongly suggests that the reason Aβ deposition in the human brain occurs first and to the greatest extent in regions of the brain overlapping with what is called the “default mode network” is due to greater metabolic and synaptic activity in these regions over a lifetime resulting in higher monomeric Aβ levels leading to earlier Aβ aggregation in these regions.
Cirrito JR, Yamada KA, Finn MB, Sloviter RS, Bales KR, May PC, Schoepp DD, Paul SM, Mennerick S, Holtzman DM. (2005) Synaptic activity regulates interstitial fluid amyloid-b levels in vivo. Neuron 48(6):913-922.
Cirrito JR, Kang J-E, Lee J, Stewart FR, Verges D, Silverio LM Bu G, Mennerick S, Holtzman DM. (2008) Endocytosis is required for synaptic activity-dependent release of amyloid-b in vivo. Neuron 58:42-51. PMCID: PMC2390913
Brody DL, Magnoni S, Schwetye KE, Spinner M, Esparza TJ, Stocchetti N, Zipfel GJ, Holtzman DM. (2008) Amyloid-β Dynamics Correlate with Neurological Status in the Injured Human Brain Science 321:1221 – 1224. PMCID: PMC2577829
Bero AW, Yan P, Roh JH, Cirrito JR, Stewart FR, Raichle ME, Lee JM, Holtzman DM. Neuronal activity regulates the regional vulnerability to amyloid-β deposition. Nat Neurosci. 2011 14(6):750-6. PMCID: PMC3102784
Bidirectional relationship between sleep, amyloid-β and tau
We found that in the brain ISF in mice and in CSF in humans, Aβ is higher during wakefulness and lower during sleep. We also found that at least part of these fluctuations are due to differences in synaptic activity and Aβ generation between the wake and sleep states. Increasing wakefulness by sleep deprivation and with orexin both acutely increased Aβ as well as chronically increased Aβ deposition. Increasing sleep with an orexin receptor antagonist acutely decreased Aβ and chronically decreased Aβ deposition. We also found that genetic manipulation of orexin produces similar results and that this effect appears to be due to the effect of orexin on sleep. Once Aβ deposition occurs, this results in disrupted sleep and even greater Aβ production. Recently, we have also found that wakefulness and sleep deprivation regulates the release of extracellular tau and tau spreading in both mice and humans and that tau pathology is linked with decreased NREM slow wave sleep in humans. This work has important implications for not only a mechanism of how normal brain function and its dysfunction could predispose to AD but also suggests that understanding and treating sleep disorders may provide a new way to decrease AD risk.
Kang JE, Lim MM, Bateman RJ, Lee JJ, Smyth LP, Cirrito JR, Fujiki N, Nishino S, Holtzman DM. (2009) Amyloid- β Dynamics Are Regulated by Orexin and the Sleep-Wake Cycle. Science. 326:1005-1008. PMCID:PMC2789838
Roh JH, Huang Y, Bero AW, Kasten T, Stewart FR, Bateman RJ, Holtzman DM. Disruption of the sleep-wake cycle and diurnal fluctuation of amyloid-β in mice with Alzheimer’s disease pathology. Science Translational Medicine 2012 Sep 5;4(150):150ra122. PMCID: PMC3654377
Lucey BP, McCullough A, Landsness EC, Toedebusch CD, McLeland JS, Zaza AM, Fagan AM, McCue L, Xiong C, Morris JC, Benzinger TLS, Holtzman DM. Reduced non-rapid eye movement sleep is associated with tau pathology in early Alzheimer’s disease. Sci Transl Med. 2019 Jan 9;11(474). pii: eaau6550. doi: 10.1126/scitranslmed.aau6550.
Holth JK, Fritschi SK, Wang C, Pedersen NP, Cirrito JR, Mahan TE, Finn MB, Manis M, Geerling JC, Fuller PM, Lucey BP, Holtzman DM. The sleep-wake cycle regulates brain interstitial fluid tau in mice and CSF tau in humans. Science 2019 Feb 22;363(6429):880-884. doi: 10.1126/science.aav2546. PMCID:PMC6410369
Tau, ApoE, TREM-2 and the brain’s innate immune response
Increasing evidence suggests that tau pathology spreads from initially affected regions to synaptically connected regions. We hypothesized that if the spreading of tau pathology occurs in a prion-like manner extracellularly, then studying extracellular tau metabolism would provide insight into what regulates the levels of both monomeric and oligomeric tau in the brain ISF and ultimately the spread of tau pathology. We rationalized that certain antibodies to tau might be able to block tau pathology and neurodegeneration bind binding to extracellular forms of tau to block tau spreading. We have been able to characterize ISF tau metabolism utilizing in vivo microdialysis and what influences ISF tau levels. We found that excitatory synaptic activity increases extracellular tau in vivo. We also found that certain anti-tau antibodies that were able to block cellular tau seeding in vitro in collaboration with the Diamond lab. When tested in vivo, the antibody that best blocked tau seeding was most potent at suppressing tau pathology, decreasing brain atrophy, and improving behavior when administered either centrally or peripherally. One of these antibodies has now been humanized and is in clinical trials in tauopathies. In recent work, we have found that microglia are key effectors of both Aβ-induced tau spreading in the brain as well as tau-mediated neurodegeneration. Key molecules that influence these effects of microglia are TREM2 and apoE.
Yamada K, Holth JK, Liao F, Stewart FR, Mahan TE, Jiang H, Cirrito JR, Patel TK, Hochgräfe K, Mandelkow EM, Holtzman DM. Neuronal activity regulates extracellular tau in vivo. J Exp Med. 2014 10;211(3):387-93. PMCID: PMC3949564
Yanamandra K, Kfoury N, Jiang H, Mahan TE, Ma S, Maloney SE, Wozniak DF, Marc, Diamond MI, Holtzman DM. Anti-tau antibodies that block tau aggregate seeding in vitro markedly decrease pathology and improve cognition in vivo. Neuron 2013 Oct 16;80(2):402-14. PMCID: PMC3924573
Leyns CEG, Gratuze M, Narasimhan S, Jain N, Koscal LJ, Jiang H, Manis M, Colonna M, Lee VMY, Ulrich JD, Holtzman DM. TREM2 function impedes tau seeding in neuritic plaques. Nat Neurosci. 2019 Aug;22(8):1217-1222. doi: 10.1038/s41593-019-0433-0. PMCID:PMC6660358
Shi Y, Manis M, Long J, Wang K, Sullivan PM, Remolina Serrano J, Hoyle R, Holtzman DM. (2019) Microglia drive APOE-dependent neurodegeneration in a tauopathy mouse model. J Exp Med. 216(11):2546-2561. PMCID: PMC6829593